Description:
This unusual creature was first thought when described in 1979 to
be a polychaete worm. Recent specimens with more detail have shown
it to have affinities with the unusual stem-group arthropod Opabinia
regalis of the slightly younger Burgess Shale. Myoscolex is unusual
in that the preservation is in apatite (calcium phosphate) rather
than calcium carbonate as are most Emu Bay fossils. What are actually
seen here are the muscle blocks separated by segmented boundaries,
along with the outlines of the lobopodous segments themselves. These
specimens show the oldest known instances of muscle tissue yet recorded,
making them most unusual. During decay of muscle tissue, phosphates
and fatty acids create a low-pH microenvironment. When such decay
takes place in the absence of oxygen over several weeks, deposition
of calcium phospahte can occur. This is a truly exceptional specimen,
and as such is quite uncommon. I have included a reconstruction
of the taxon showing what they looked like in life. The tip of the
“trunk” and the tailfan have been restored after Opabinia.
The Emu Bay Shale is Australia’s only Burgess Shale type deposit.
It has fauna in common with both the younger Burgess Shale and the
slightly older Chengjiang Biota. For this reason I have chosen to
place this item in the Chengjiang Biota section. Unlike the Burgess
Shale and Chengjiang Biota, however, the Emu Bay Shale deposits
were a shallow water depositional environment. Perhaps the combination
of shallow water and anoxia is what led to the unusual muscle preservation
seen here. Whatever the case, this is a most unique offering, indeed!